Water-assisted ground state intra-molecular proton transfer in 2,5-dihydroxy-substituted azobenzenes: experimental and computational studies

Article abstract of DOI:10.1039/C8CE01878B

Herein, the phenomenon of ground-state proton transfer in a series of 2,5-dihydroxy azobenzene derivatives was studied. The mechanism of proton transfer in a dimethylacetamide (DMA)-water mixture was investigated by UV-vis spectroscopy. Time-dependent den

Full text of DOI:10.1039/C8CE01878B

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DOI: 10.1039/C8CE01878B
Journal Name
ARTICLE
Water-assisted Ground State Intra-molecular Proton Transfer in
2,5-dihydroxy-Substituted Azobenzenes: Experimental and
Computational Studies
Received 00th January 20xx,
Accepted 00th January 20xx
Dhiraj Dasa and Angshuman RoyChoudhury*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The phenomenon of ground-state proton transfer in a series of2,5-dihydroxy azobenzenes derivatives has been studied.
The mechanism for proton transfer in dimethylacetamide (DMA)-water mixture has been investigated by UV-Vis
Spectroscopy. The Time-Dependent Density Functional Theory (TD-DFT) calculation was performed to get further
confirmation. The intramolecular hydrogen bond between the hydrogen of the o-hydroxy group and azo-group is found to
be reinforced in the hydrogen transfer process. The topological properties of the intramolecular interactions have been
determined by atoms in molecules (AIM) calculations with the help of AIM2000 package. It has been observed that water
and substitutions in the ortho-position corresponding to azo-group has facilitated the proton transfer mechanism to occur
in the ground state. The effect of substitutions and water molecule has been investigated by DFT calculations.
and theoretical calculations (DFT).21 Ozenet al. have studied the
effect of cooperative hydrogen bonding in keto/enol tautomerism
of hydroxyl-azo dyes by theoretical calculations (DFT).22
Introduction
Functionalized azobenzenes(ABs) represent an important class of
compounds mainly because of their photo-switching properties and
therefore they are used as molecular devices.1-3 A prominent
feature of hydroxy-azobenzenes (OH-ABs) is keto/enol tautomerism
(Scheme 1). This tautomerism has been exploited towards signalling
and switching.4Hydroxy group, close to the azo group (-N=N-),
shows hydrogen atom-coupled electron transfer (PCET) as
well.5,6PCET reactions have importance in many biological processes
and small molecule activations related to the conversion and
storage of solar energy, mainly carbon dioxide reduction and water
oxidation.7-10 Further, the OH-ABs exhibits ground/excited state
proton transfer through hydrogen bonding. Intra-molecular and
intermolecular proton transfer have gained great interest in
chemistry, physics, and biology due to their application as
fluorescence chemo-sensors, laser-dyes, and molecular-switches.11-
Herein, we have synthesized four derivatives of dihydroxy-
azobenzene (1-4) (Scheme 2) and studied intramolecular proton
transfer in the ground state. We have structurally characterized the
intermediate for enol/keto tautomerism for the first time. Three
keto forms among four compounds have been structurally
characterized along with enol forms. Many groups have suggested
that water molecules and hydrogen bonding are the initiators for
the proton transfer. We have validated such explanations by
structural chemistry and computations.
N
N
N
N
N
H
N
H
N
N
or
or
O
O
HO
OH
keto form
enol form
Scheme 1 Keto/enol tautomerism in OH-ABs.
N
N
N
N
14
N
The hydrogen bonding interactions with the solvent molecules
R1
H
R1
N
O
OH
HO
OH
R1
play important role in this proton transfer reaction. Many groups
have studied proton transfer mechanism spectroscopically and by
theoretical calculations.15-20 They suggested that hydrogen bonding
and water molecules assist these proton transfers in the excited
state or ground state. Wachtveitl et al. have studied proton transfer
mechanism on OH-ABs in water by ultrafast spectroscopic studies
HO
OH
keto form
enol form
R1 = NO2 (1), CN (2), Cl (3) Br (4)
R1 = NO2 (1), CN (2), Cl (3) Br (4)
(a)
(b)
Scheme 2 (a) Dihydroxy azo-benzene derivatives and (b) keto/enol
tautomerism.
Experimental section
Starting Materials
All the compounds were purchased from various commercial
suppliers and were used as received. Solvents and reagents were
purchased from Merck chemicals, India, Sigma-Aldrich and used as
received.
a. Department of Chemical Sciences, Indian Institute of Science Education and
Research Mohali, Sector 81, S. A. S. Nagar, Manuali PO, Mohali, 140306, Punjab,
India. E-mail: angshurc@iisermohali.ac.in; Fax: +911722293167
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
Table 1.Crystallographic and Refinement Data of all the compounds.
View Article Online
DOI: 10.1039/C8CE01878B
Compound ID
CCDC number
Formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a, Å
b, Å
c, Å
α (°)
β (°)
1k
2e
3e
3k
4k
2-int
1876824
C13H9N3O2
239.23
1876826
C12H9N3O4
259.22
0.42×0.41× 0.4
monoclinic
P21/n
11.8236(6)
8.6618(3)
12.0259(6)
90
113.493(6)
90
1876827
C16H16N4O3
312.33
0.52×0.4× 0.22
triclinic
1876822
C16H18ClN3O3
336.06
0.34×0.32× 0.28
triclinic
1876823
C12H11ClN2O3
266.68
0.30×0.26×0.20
monoclinic
P21/n
3.9592(2)
13.8353(7)
22.1033(11)
90
93.241(5)
90
1876825
C12H11BrN2O3
311.14
0.2×0.15×0.12
monoclinic
P21/n
3.93930(10)
13.8586(4)
21.9545(7)
90
93.737(3)
90
1196.02(6)
0.32×0.12×0.09
monoclinic
P21/c
10.0695(11)
4.4780(5)
23.887(3)
90
92.333(3)
90
1076.2(2)
1
P
1
P
7.0323(3)
8.6204(4)
13.7182(6)
105.738(2)
94.770(2)
103.820(2)
767.47(6)
7.0839(3)
10.8648(6)
10.9553(6)
90.615(5)
94.232(4)
107.982(5)
799.30(7)
γ (°)
V, Å3
1129.53(10)
1208.81(11)
Z
4
2
2
4
4
4
Z'
1
1
1
1
1
1
ρcalc(g cm-3)
1.524
1.352
1.396
1.465
1.728
1.476
μ (mm-1)
0.118
0.096
0.258
0.318
3.439
0.104
Temperature (K)
2Θmin, max
100.0(2)
5.98-49.97
536.0
100.0(2)
3.124-49.994
328.0
100.0(2)
5.332-49.982
352.0
298.0(2)
5.89-49.994
552.0
100.0(2)
5.88-49.988
624.0
100.0(2)
4.048-53.044
496.0
F (000)
No. of Reflections 6344
10582
9552
7717
8806
15398
collected
Rint
0.0166
unique 1984
0.0167
2711
0.0242
2799
0.0428
2131
0.0341
2110
0.0489
2226
No.
of
reflections
Data/restraints/param
eters
1984/0/180
2711/0/218
2799/0/219
2131/0/179
2110/0/179
2226/0/171
Diffractometer
Detector
and Rigaku XtaLAB Bruker Kappa Rigaku XtaLAB mini Rigaku XtaLAB Rigaku
mini and Apex II and CCD and Mercury375/M mini and mini
Mercury375/M Mercury375/M
XtaLAB Bruker
D8
and
and Venture
Mercury375/M
CCD
CCD
Photon 100 CMOS
CCD
CCD
R1 [I>2σ(I)]
wR2 (all data)
GooF on F2
0.0363
0.0324
0.1020
1.099
0.0487
0.1365
1.098
0.0580
0.0309
0.0510
0.1544
1.087
0.0983
1.077
0.1889
0.0761
1.027
1.102
Largest
diff. 0.20/-0.23
0.23/-0.23
0.53/-0.25
0.54/-0.23
0.81/-0.37
0.22/-0.29
Peak/hole/e (Å3)
Index ranges
-12 ≤ h ≤ 14
-10 ≤ k ≤ 10
-14 ≤ l ≤ 14
-8 ≤ h ≤ 8
-8 ≤ k ≤ 10
-16 ≤ l ≤ 16
-8 ≤ h ≤ 8
-12 ≤ k ≤ 12
-13 ≤ l ≤ 13
-4 ≤ h ≤ 4
-16 ≤ k ≤ 16
-26 ≤ l ≤ 24
-4 ≤ h ≤ 4
-16 ≤ k ≤ 16
-26 ≤ l ≤ 21
-12 ≤ h ≤ 12
-5 ≤ k ≤ 5
-29 ≤ l ≤ 29
2 | J. Name., 2012, 00, 1-3
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Table 2 Bond distances
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DOI: 10.1039/C8CE01878B
Bond
Distances (Å)
Bond
Distances (Å)
1.281(2)
1.390(3)
1.433(3)
1.351(3)
1.3979(18)
1.3189(16)
1.3254(18)
1.472(2)
1.2788(16)
1.2752(15)
1.3898(16)
1.4274(17)
1.3481(16)
1.279(2)
C7N2
N2N1
N1C1
C1C6
C6O1
N1N2
N2-C8
C8C9
C9O1
N1N2
N1N2
N1C6
C6C1
C1O1
1k
3e
1.306(3)
1.341(3)
1.456(4)
1.292(3)
1.301(3)
1.344(4)
1.459(4)
1.298(4)
N1N2
N2C7
C7C8
C8O2
N1N2
N2C7
C7C8
C8O2
2e
3k
4k
1.393(3)
1.427(3)
1.344(3)
N1C1
C1C6
C6O1
2-int
Table 3. Hydrogen bond distances of all the compounds
Donor (D)-H...Acceptor(A)
D-H (Å)
0.95(2)
0.92(2)
0.92(2)
0.87(2)
H..A (Å)
1.64(2)
1.85(2)
2.033(18)
1.81(2)
D..A (Å)
Symmetry
D-H..A (°)
2.5837(16)
2.5802(16)
2.6288(16)
2.5860(13)
176.2(18)
135.3(15)
121.(16)
149(2)
½ -x,y-½,½ -z
O2H2···O1
Intramolecular
Intramolecular
intramolecular
1k
2e
N2H2A···O1
N2H2A···O3
O1H1···N1
0.87(2)
0.92(3)
2.44(2)
1.68(3)
2.9514(14)
2.5809(14)
118(2)
Intramolecular
Intramolecular
O1H1···N2
O2H2···O3
165.9(17)
0.95
0.95
0.95
1.14(3)
2.58
2.54
2.59
2.28(3)
3.4743(18)
3.4833(15)
3.4885(15)
2.929(2)
157
175
158
114(2)
-x,-y,1-z
1-x,2-y,1-z
1-x,1-y,-z
C3H3···N3
C5H5···O3
C10H10···O2
O1H1···N1
Intramolecular
1.14(3)
0.92(3)
1.52(3)
2.00(3)
2.541(2)
2.900(3)
146(3)
167(3)
Intramolecular
O1H1···N2
O2H2···N3
2-int
1-x,-y,1-z
0.95
0.95
0.91(3)
0.91(3)
2.39
2.51
2.40(4)
1.74(3)
3.251(3)
3.286(3)
2.933(2)
2.555(2)
150
139
117(3)
148(3)
1-x,-y,1-z
1+x,2+y,z
Intramolecular
Intramolecular
C5H5···O1
C10H10···O2
O1H1···N1
O1H1···N2
3e
3k
0.78(3)
1.84(4)
2.619(2)
177(5)
Intermolecular
O2H2···O3
0.93
2.53
3.183(2)
2.614(3)
2.941(2)
2.539(3)
2.826(3)
2.773(3)
3.064(2)
2.551(3)
2.603(3)
2.743(3)
2.816(3)
128
Intermolecular
1-x,1-y,1-z
Intramolecular
Intramolecular
Intramolecular
1+x,y,z
C2H2A···O3
O1H1···O3
0.94(4)
1.02(4)
1.02(4)
0.90(4)
0.99(6)
0.77(4)
0.77(4)
0.86(4)
0.91(5)
0.91(4)
1.69(4)
2.46(4)
1.75(4)
1.94(4)
1.82(5)
2.68(3)
1.90(4)
1.74(4)
1.86(5)
1.93(4)
168(4)
109(3)
132(3)
168(4)
163(5)
113(3)
142(3)
175(4)
165(5)
165(4)
N1H(1A)···Cl1
N1H(1A)···O2
O3H(3A)···O2
O3H(3B)···O2
N1H(1A)···Br1
N1H(1A)···O2
O1H1···O3
Intramolecular
Intramolecular
4k
1-x,1-y,1-z
-1+x,y,z
intramolecular
O3H(3A)···O2
O3H(3B)···O2
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Synthesis and Characterization
The crystal structure of 1 (crystallized from EtOH) shows keto form
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of this molecule, named as 1k (k = keto) (FigDuOreI:11a0).1.0It39c/rCy8stCaEll0iz1e87s8iBn
monoclinic space group P21/n. The solid-state structure shows that
the -OH hydrogen atom (H2A) of resorcinol moiety (C6-O1) near to
azo group (-N=N-) has got transferred to the nitrogen atom (N2)
푹ퟏ(ퟔ)
Since the method of the synthesis of all the compounds is the same,
only a representative procedure is described below.
All the compounds were synthesized using the procedure
reported in the literature.23An ice-cooled solution of sodium nitrite
(NaNO2, 4.1 mmol) in water was added to the solution of amine(4
mmol) in dilute hydrochloric acid (6N, 8 mL) at 0-5 °C and then the
mixture was stirred for half an hour. After that this cooled
diazonium salt solution was added to the solution of resorcinol (4
mmol), sodium hydroxide (NaOH, 12 mmol) and sodium carbonate
(Na2CO3, 12 mmol) in water at 0-5 °C. Then mixtures were stirred
for 1 hour at 0-5 °C. After that, the mixtures were filtered and then
the residues were washed with water and dissolved in different
solvents for crystallization.
and thus formed an intramolecular six-member ring (
) closed
ퟏ
by a weak intramolecular hydrogen bonding interaction between
N(2)-H(2A) and O1. Because of proton transfer to the N2, the N-N
distance [1.3181(15) Å] is notably longer than normal azo-bond
distances (-N=N-, 1.24-1.28Å) resulting into the formation of the
keto form. Due to the formation of a six-member ring by O1-C6-C1-
N1-N2-H(2A), the bond distances related to this ring are quite
perturbed than their normal reported values as listed in Table S2.
The N-H hydrogen atom (H2A) is also hydrogen bonded with an
oxygen atom (O3) of nitro group forming another six-member ring (
푹ퟏ(ퟔ)
). The AIM analysis (vide infra) shows that, there exist BCPs
ퟏ
Crystallization
and RCPs formed by these intramolecular hydrogen bonds. In the
crystal packing, there exists intermolecular hydrogen bonding
between the O1 and hydrogen atom {H2' (' = ½ -x, ½ +y, ½ -z } of OH
group of another symmetry related azo-molecule and formed 1D
chain (Figure 1b) along crystallographic b direction. The details of
hydrogen bonding distances and angles are represented in Table 3.
All are the compounds were recrystallized using different solvents.
Suitable single crystals for X-ray diffraction were obtained from
ethanol (EtOH), dimethylformamide (DMF), dimethylacetamide
(DMA) and tetrahydrofuran (THF)-methanol (MeOH) mixture for the
compound 1{(E)-4-((2-nitrophenyl)diazenyl)benzene-1,3-diol}, 2{(E)-
2-((2,4-dihydroxyphenyl)diazenyl)benzonitrile},3{(E)-4-((2-
chlorophenyl)diazenyl)benzene-1,3-diol},
bromophenyl)diazenyl)benzene-1,3-diol},respectively.
and
4{(E)-4-((2-
Suitable
single crystals for X-ray diffraction were also obtained from THF-
MeOH mixture in case of 3. The asymmetric unit of all the
compounds contains one molecule of the azo-compound. Their
crystallographic parameters are represented in Table 1. All the
aromatic hydrogen atoms were fixed at their geometrically ideal
locations and refined using riding model but the H atoms linked to
O or N atoms were located from the difference Fourier map and
were refined isotropically.
(a)
Computational Details
All the theoretical calculations have been done using Gaussian 0924
program and Gauss View 0525 has been used as a graphical
interface. The interaction energy calculations were performed at
Møller-Plesset perturbation theory (MP2) level using 6-31G(d) basis
set, diffuse and polarization functions were used for halogens. For
Topological parameter calculations, the suitable wavefunction files
were generated using Gaussian 09 and those wavefunction files
were used as input for AIM2000 package26 for analysis of
∇
topological parameters like electron densities (ρc) and Laplacian (
(b)
2ρc) of electron density at bond critical point (BCPs) and ring critical
point (RCPs). For Time-Dependent Density Functional Theory (TD-
DFT) calculations, all the geometries were optimized at MP2 level in
gas phase then single point energy calculations have been done at
DFT/B3LYP level using 6-311G(2d,p) as a basis set and diffuse and
polarization functions were used for halogens. The effect of the
solvents were modelled with the polarizable continuum model
(PCM).27 Density Functional Theory (DFT) calculations for keto/enol
tautomerism were performed at the DFT/B3LYP level using 6-
31++G(d,p) as a basis set. All the geometries including transition
states (TS) were optimized without any constraints. The
characteristics of the stationary points were analysed by vibrational
frequency calculation. All the TSs showed one negative imaginary
frequency vibrating in the directions of the reaction coordinate. All
the ∆G values were calculated at 298 K.
Fig. 1 (a) ORTEP view (with 50% ellipsoid probability) and (b)
interactions in the solid-state in 1k.
Crystal structure of 2 (grown from DMF) shows the enol form,
named as 2e (e = enol)(Figure 2a). It crystallizes in triclinic space
1
group P . The asymmetric unit (Figure 2a) contains one azo-
molecule and one DMF molecule. In the solid-state, there exist
intra- and intermolecular hydrogen bonding interactions. The OH
hydrogen atom (H1) of resorcinol moiety near to azo group (-N=N-)
is hydrogen bonded with N1 atom and formed a six-membered ring
푅1(6)
···
enclosed by this interaction (
). The O1-H1 N1 distance is
1
1.81(2) Å. Here N(1)=N(2), N2-C8, C8-C9, and C9-O1 bond distances
are as normal as reported in the literature (Table S2). The O3 of
DMF molecule is hydrogen bonded with H2 of para-substituted OH
Results and discussion
4 | J. Name., 2012, 00, 1-3
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group of azo-molecule. The AIM analysis (vide infra) of the infra) shows that the ρc at BCP for this N1....H(1)-O interaction has
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asymmetric unit exhibits BCPs and RCPs of this interactions. In been found to be 0.081 e Å-3 which is more than double than
DOI: 10.1039/C8CE01878B
addition, there exist C-H···O/N interactions and π-π stacking {Figure compound 2e (0.037 e Å-3). In addition, there exist intermolecular
2c; C7···C7' (-x, 1 - y, 1 - z) = 3.3868(17) Å and C8···C2' (1 - x, 1 - y, 1 - interactions along with π-π stacking {C1···C4' (' = x,1+y,z) = 3.296(3),
z) = 3.3623(17) Å}; and due to these interactions, it developed 1D C4···C1'' ('' = x,-1+y,z) = 3.296(3), C7···C10'' ('' = x,-1+y,z) = 3.390(3),
chain (Figure 2b) along crystallographic c and a direction, and C10···C7' (' = x,1+y,z) = 3.390(3)} in the solid-state.
respectively. The interactions have been reported in the Table 3.
The structural characterization of 3 (crystallized from DMA) shows
enol form of it named as 3e (Figure 4a). It crystallizes in triclinic
1
system with space group P . The asymmetric unit of it contains one
molecule of azo-molecule and one DMA molecule and are
connected through hydrogen bonds in the lattice. In the solid-state,
there exist intra- and intermolecular hydrogen bonding
interactions. The OH hydrogen atom (H1) of resorcinol moiety near
to azo group (-N=N-) is hydrogen bonded with N2 atom and formed
푅1(6)
(a)
a six-membered ring enclosed by this interaction (
). The O1-
1
H1...N2 distance is 1.74(3) Å. The AIM analysis (vide infra) of the
asymmetric unit exhibits BCPs and RCPs of this interaction. Here
N(1)=N(2), N1-C6, C6-C1, and C1-O1 bond distances are as normal
as reported in the literature (Table S2). In addition, there exist
intermolecular hydrogen bonding interactions involving C-H···O/Cl
interactions and as a result, it developed a hydrogen bonded 2D
network in the solid-state (Figure 4b and 4c). The hydrogen bonding
distances are represented in Table 3. There also exists C-H···π
interaction {C1···H13A' (' = 1-x,1-y,1-z) = 2.80 Å and C9···C15A'' ('' =
x,y,-1+z) = 2.88 Å}.
(b)
(c)
(a) ORTEP view(with 50% ellipsoid probability), (b)
interactions in solid-state, and (c) π-π stacking in 2e.
Fig.
2
(a)
Fig. 3 ORTEP view of 2-int
(b)
We have also crystallized 2 from THF-acetonitrile (ACN) mixture
named as 2-int (int = intermediate) (Figure 3). It crystallizes in
monoclinic space group P21/c. The asymmetric unit contains one
azo-molecule only. Here the ortho-hydroxy group is hydrogen (H1)
bonded with azo nitrogen (N2). But interestingly the O-H distance is
1.14(3) Å and the hydrogen (H1) is closer to azo nitrogen (N1)
where the hydrogen bonding distance is 1.52(3) Å forming an
푅1(6)
intramolecular hydrogen bonded ring (
). AIM calculation (vide
1
(c)
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Fig.
4 (a) ORTEP view(with 50% ellipsoid probability), (b)
View Article Online
DOI: 10.1039/C8CE01878B
interactions in the solid-state, and (c) C-H··π stacking in 3e.
The structural characterization of 3 and 4 from THF-MeOH mixture
shows keto form of those, named as 3k and 4k, respectively (Figure
5a and 5b). Both the compounds crystallize in monoclinic space
group P21/n and they are isomorphous. As the structure 4k was
solved and refined using a data collected at 100.0(2) K, the
structural features of 4k will be discussed here. The asymmetric
unit of both contains one azo-molecule and one water molecule
and they are hydrogen bonded to each other. The solid-state
structure exhibits that the OH hydrogen atom (H1A) of resorcinol
moiety near to azo group (-N=N-) has got transferred to the
(d)
Fig. 5 ORTEP view(with 50% ellipsoid probability) of (a) 4k, (b) 3k,
and interactions in the solid-state of (c) 4k, (d) 3k.
2. AIM calculations
푅1(6)
The intra- and intermolecular interactions with lattice solvent
molecules have been calculated using the AIM approach after
optimizing the molecular conformation from the observed crystal
structure. The calculations confirm the existence of BCP between
the H atom and acceptor atom (N or O) and RCP has been also
found in case of intramolecular interactions (Table 4). AIM analyses
indicate that these interactions are of hydrogen bond type.
nitrogen atom (N1) and has formed a six-membered ring (
1
).enclosed by weak hydrogen bond interaction between N1-H(1A)
and O2 where the distance is 1.90(4) Å. Because of hydrogen atom
transfer to N1, the N-N distance [1.301(3) Å] is longer than normal
azo-bond distances (Table S2). Due to six-member ring formation,
the bond distances related to this interaction are perturbed than
their normal literature reported values (Table S2). The O2 is
hydrogen bonded with H3B of water molecule where the distance is
1.93(4) Å. The AIM analysis (vide infra) has been done using the
asymmetric unit and it shows that there exist BCPs and RCPs
formed by the intra-molecular interactions. In the crystal packing,
there exist intermolecular hydrogen bonding between the O3 of
water and hydrogen atom {H1' (' = 1-x,1-y,1-z)} ( Table 3) of para-
OH group of another symmetry related azo-molecule and formed
1D chain (Figure 5c and 5d) along crystallographic a direction.
Table 4 AIM analysis for the compounds
2
2
∇
∇
(ρc)
ρc
Interactions (at BCP)
(e Å-3)
ρc
(ρc)
(at RCP)
(e Å-3)
0.018
0.014
0.018
(at BCP)
(e Å-5)
0.010
0.005
0.008
(at RCP)
(e Å-5)
0.007
0.005
0.007
N2-H....O1
N2-H....O3
0.047
0.022
0.046
1k
2e N1....H-O1
2-
int
N2....H-O1
0.042
0.007
0.017
0.006
3e N2....H-O1
3k N1-H....O2
4k N1-H....O2
0.051
0.059
0.058
0.009
0.012
0.012
0.019
0.020
0.020
0.007
0.008
0.008
3. Proton transfer study
The UV-Vis spectra (ESI, Fig. S1) of the azo-compounds were
recorded in dimethylacetamide (DMA) solvent using Lab India UV
3200 double beam UV-Vis spectrophotometer. The UV-Vis spectra
exhibit mainly two peaks, one at ~400 nm characteristic for π→π*
transition and another is at ~500 nm characteristic for n→π*
transition. The crystal structure of 3k contains one water molecule,
which is hydrogen bonded to O2 and the hydrogen atom (H1A) has
been transferred to the N1 of the azo-group. So, this structure
motivated us to study proton transfer in the ground state and
directed us to think that the water molecule has a significant role in
this transformation. Thus, we have recorded the UV-Vis spectra
(ESI, Fig. S2) of all the compounds in DMA in the presence of water.
The analysis of the spectra reveals that on increasing percentage of
water, the peak for enol tautomer decreased and that for keto has
increased. This could be possible if phenolic hydrogen (ortho- to the
azo group) has been transferred to the nitrogen atom of the azo
group. As the hydrogen atom is coordinated to the lone pair of the
nitrogen atom, it may result in the decrease of n→π* transition
peak. To get further confirmation, we have done TD-DFT
calculations and it shows that the oscillator strengths (f) have been
(a)
(b)
(c)
6 | J. Name., 2012, 00, 1-3
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ARTICLE
decreased in the keto form for n→π* transition (Table 5). But in the
case of 2, there are no changes in n→π* transition. Then we have
looked into the transition were π-orbital of azo-group is involved.
There it exhibits a decrease of oscillator strengths (f) in the water
medium (Table 6).
In case of 1, it shows negative values for ∆G (-1.31 kcal/mol), ∆H (-
View Article Online
1.35 kcal/mol) and ∆E (-1.35 kcal/mol) forDtOhIe: 1f0o.1r0m3a9/tCio8nCEo0f18k7e8tBo
form. Thus, it favoured the proton transfer process and the
formation of the keto form. In addition, the structural analysis
shows that there is a hydrogen bond between oxygen (O3) of the
nitro group and N-H hydrogen atom (H2A). Thus, it reveals that the
hydrogen bond with the substitution favoured the proton transfer
process and the formation of keto form without the involvement of
water molecule.
Table 5 Calculated Oscillator Strengths (f) of the compounds in DMA
and Water.
The Peak for nπ*
transition (nm)
429.33
Oscillator Strengths
(f)
1 in DMA (1DMA)
1 in Water (1W)
2 in DMA (2DMA)
2 in Water (2W)
3 in DMA (3DMA)
3 in Water (3W)
4 in DMA (4DMA)
4 in Water (4W)
0.2047
0.0017
0.0002
0.0002
0.5756
0.0054
0.1183
0.0016
Conclusions
441.38
430.9
432.8
398.44
420.86
427.68
421.05
Ground state proton transfer mechanism has been studied
both experimentally and computationally. We have
structurally characterized the intermediate for proton transfer
by single crystal X-ray diffraction and experimentally
demonstrated the same by UV-Vis spectroscopy. Further
confirmation has been arrived at by computations. The
hydrogen bonding and substitutions close to the azo group has
monitored the proton transfer in the ground state. The
interaction of the water molecule with the phenolic oxygen
has weakened the O-H bond and strengthened the H-N bond.
The effect of the substitutions is also similar like water in this
proton transfer mechanism. The large size substitution has a
large effect for this transformation. In the case of –NO2, the
hydrogen atom has been transferred totally to the nitrogen
atom of the azo group. But in the case of –CN, the O-H bond is
quite elongated towards the azo- group.
Table 6 Calculated Oscillator Strengths (f) of 2 in DMA and Water.
Peak involved for this Oscillator
transitions (nm)
Strengths (f)
2
in
(2DMA)
2 in Water (2W) 447.40
DMA 407.92
0.6582
0.2495
3k and 4k contain one water molecule in the asymmetric unit
where 1k does not have any lattice solvent molecule but still,
transfer of hydrogen atoms was observed. Thus, to get insight
about the role of substitutions and water molecule, DFT calculation
has been performed. All the values obtained by theoretical
calculation have been represented in the ESI Table S1a, 1b, 1c, and
1d for compound 1, 2, 3, and 4, respectively.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
For this study, we have first considered the compound 3 as we were
able to structurally characterize both enol (3e) and keto (3k) form,
and 3k contains one lattice water molecule (O3) which is hydrogen
bonded with keto carbonyl (O2). We have optimized the enol, keto,
and transition state (TS) for all compounds without any constraints.
First, we have calculated the thermodynamic parameters (∆G, ∆H
and ∆E) for 3 without including the water molecule in the system. It
reveals that enthalpy of formation (∆H = 0.68 kcal/mol) and
formation energy in term of electronic energy (∆E = 0.68 kcal/mol)
of keto form are positive. But when we have considered the water
molecule in the system then it reduced the ∆H (-2.85 kcal/mol) and
∆E (-2.26 kcal/mol) values to negative. Similar results were also
obtained in the case of 4. Thus, we can infer that the water
molecule favoured the proton transfer process and the formation of
keto form in the presence of water in the medium. In the case of 2,
we found positive values for ∆H and ∆E where water molecule has
not been included. When we have included the water molecule in
the system then it also shows negative values for ∆H and ∆E even
for the formation of keto form. We have tried to get the structure
of the keto form of 2 from THF-ACN mixture and structural
characterization of it (2-int) shows that O-H distance is 1.14(3) Å,
which is closer to a transition state.
DD thanks IISER Mohali for post-doctoral fellowship. All
financial, instrumental (UV-Vis, NMR, PXRD, SCXRD, DSC etc.)
and infrastructural support from IISER Mohali is gratefully
acknowledged. We thank Dr.Parthapratim Munshi for single
crystal X-ray diffraction of 2-int.
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View Article Online
DOI: 10.1039/C8CE01878B
N
O
N
H
R
HO
TS
N
N
O
N
N
H
R
Keto form
HO
R
Enol form
HO
OH
R = NO2, CN, Cl, Br